The Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) standards specify rates and formats for synchronous digital optical communication networks intended for use in telephony and at the lowest layer of Integrated Services Digital Networks (ISDNs). They establish a hierarchy of data transfer rates extending from a level that might be required by a small group of users (51.84 Mb/s), to a level that is suitable for long-haul carrier operations (2.488 Gb/s). The expected uses of the networks demand provisions for multiplexing lower-rate signals together into a higher rate signal for long-distance transmission, monitoring system performance to ensure reliability, and communicating maintenance information among nodes in the network without interrupting normal service. The synchronous nature of the network necessitates a means of extracting timing information from an incoming data stream and recognizing timing failures. The standards answer these concerns by establishing a frame structure within the transmitted signal and dividing each frame into a payload and an overhead section. The frame structure supports multiplexing by allowing high rate signals to be specified in terms of combinations of lower rate signals. The payload section is filled with the data to be transmitted across the network, while the overhead section allows the insertion of synchronization strings, parity check bytes, and maintenance communication channels.
Different elements in the transmission network need access to different amounts of knowledge about the data being transmitted. An end receiver, for instance, must establish synchronization with the frame structure of the incoming signal, check it for data integrity, process any maintenance messages that may accompany it, pass information to its companion transmitter, and extract the payload data. A mid-network regenerator need only synchronize with the data, check for any signal error conditions, and re-transmit the data. A multiplexer lies somewhere in between.
The standards codify these differences by establishing four layers of access to the SONET/SDH stream: photonic (or physical), section, line, and path. An element that changes the stream at one of these levels is called a terminating element of that layer, and is required to interpret the associated overhead.
Referring to FIG. 1, a SONET 2.488 Gb/s (OC-48) signal 20 is graphically represented showing its frame structure and the named section overhead bytes 22 and line overhead bytes 24.
Conceptually, the frame of any higher-rate signal in the SONET hierarchy is built by byte-interleaving frames of 51.84 Mb/s (OC-1) signals. Thus an OC-48 frame 20 can be depicted as a three-dimensional structure of depth 28 of 48 tiers, where each tier is an OC-1 frame 28. An OC-1 frame 28 is transmitted one byte at a time, stepping along the rows 30 (i.e. A1, A2, C1, . . . , B1, E1, F1, . . . ), so the byte interleaving process used to construct higher-rate frames results in a transmission indexed first by depth 26, then along the rows 30 (i.e. A1, A1, . . . , A2, A2, . . . ). This formalization allows the time duration of a SONET frame 20 to be the same at every rate, 125 .mu.s, and ensures that the bytes of the framing pattern (A1, A2) are always transmitted at the beginning of the frame.
Many of the overhead bytes 22, 24 are defined only for the first OC-1 tier 28a of a higher rate signal; for instance, the byte behind the B1 byte referred to by 32 serves no defined function. Other overhead bytes are defined for all tiers. In the following discussion, it is assumed that an overhead byte is defined unless otherwise explicitly noted.
The section overhead 22 consists of the named bytes A1, A2, C1, B1, E1, F1, and D1-D3. The A1 and A2 bytes together form the pattern by which the beginning of a frame can be recognized, and as such are defined for all 48 OC-1s 28 of an OC-48 signal 20. They always hold the same values: A1=F6, A2=28 in hexadecimal notation. The C1 byte was originally designated as an OC-1 identification byte, defined for all OC-1s, but is currently under study for use as a section trace indicator to identify physical connections, defined only for the first 16 tiers 28. The B1 byte is used to perform a single eight-bit bit-interleaved-parity error check covering the entirety of the previous OC-48 frame. The E1 byte carries the section orderwire channel, defined as a 64 kb/s serial channel (8 bits per frame, 8000 frames per second) for network equipment communications. The F1 byte is designated for use as a 64 kb/s user channel for implementation-specific purposes. The D1-D3 bytes form the section data communication channel, a 192 kb/s control channel for section layer network elements.
The line overhead 24 consists of the H1-H3, B2, K1-K2, D4-D12, Z1-Z2, and E2 bytes. Bytes H1, H2, and H3 are defined for all 48 tiers of the frame 20 and are used as a pointer to, and a stuff byte for, the payload data 34. The B2 byte is also defined for all 48 tiers 28; each of these 48 bytes serves as an eight-bit bit-interleaved-parity checksum for the line overhead 24 and payload envelope 34 of the STS-1 in the same tier 28 of the previous frame. The K1 and K2 bytes carry the automatic protection switching channel. The line data communication channel is carried in bytes D4-D12. It is a 576 kb/s channel analogous to the section data communication channel. The Z1 and Z2 bytes are designated as growth bytes in all 48 tiers. Only two have thus far been explicitly defined: the first Z1 byte is to be used as a synchronization status byte, redesignated S1, and the third Z2 byte, redesignated M1, is to be used to transmit a far-end-bit-error count back from remote receivers on two-way links. The E2 byte carries the line orderwire channel, a 64 kb/s channel analogous to the section orderwire.
A significant number of useful intra-network entities perform processing at the photonic, section, and line levels, but do not interact with payload data, i.e. the path level. These include all varieties of multiplexers. In addition, all elements that do manipulate payload data 34 must first resolve section and line overhead 22, 24. A processor that can provide required overhead interpretation functions at the section and line levels, as well as some signal rearrangement at those levels, is thus a general and useful element. There are a few commercially available processors that perform section and line overhead termination at SONET rates up to 622 Mb/s (OC-12), and at least one that performs limited termination functions for OC-48. No available overhead processor is available that offers signal rearrangement functions, or capture functions flexible enough to access new overhead bytes as the standards define them.
In the interest of brevity, only SONET terminology will be used in most descriptions. Unless specifically noted, the discussion also applies to the corresponding SDH requirements. The abbreviations STS-N and OC-N are used to refer to the (possibly parallelized) SONET-derived stream with a total data rate of N times the base rate of 51.840 Mb/s. Thus OC-48 and STS-48 refer to a 2.488 Gb/s stream, OC-12 and STS-12 to a 622.080 Mb/s stream, and so on. This notation implies SONET framing conventions.